1. Technical Field of the Invention
The present invention relates to a modulation device using frequency-shift keying for RF transmission of binary data. The present invention also relates to an RF transmission apparatus that incorporates such a device.
2. Description of Related Art
Frequency-shift modulation, or ‘keying’ (FSK), is frequently used for the wireless transmission of data, especially for short-range transmissions. Indeed, transmission devices that operate according to this principle are simple and inexpensive. In particular, cordless computer mice and keyboards transmit data according to this principle to a central unit which is equipped with a suitable radio receiver. This receiver can be connected to a USB port of the central unit, for example. The RF channel that is most often used for this type of transmission is situated within the standardized 27 MHz (MegaHertz) band. The channels defined inside this band are separated by 50 kHz (kiloHertz) and are: 26.945 MHz, 26.995 MHz, 27.045 MHz, 27.095 MHz, 27.145 MHz and 27.195 MHz.
For this purpose, the wireless mouse or keyboard incorporates a frequency-shift modulation device. Such a device converts a serial digital data signal into a frequency-coded RF transmission signal. Within this RF transmission signal, two signal components which have respective frequencies shifted by different amounts with respect to the RF transmission channel used correspond to the binary values of the data signal.
One of the most common ways of forming a frequency-shift modulation device implements an oscillating circuit with a crystal oscillator, whose oscillation frequency can be modified by adding a capacitor to the oscillating circuit. A micro-controller unit, or MCU, produces the binary data signal which is composed of a series of 0s and 1s, and this signal is used to control a relay for switching the additional capacitor. FIG. 1 illustrates the operational principle of an apparatus equipped with such an RF transmission device. In this figure, the references 40, 100, 50 and 60 respectively denote the micro-controller that produces the binary data to be transmitted, the frequency-shift modulation device that converts this data signal into an RF transmission signal, a filter and an RF transmitter. In a known manner, the transmitter 60 itself comprises an amplifier 61 and a transmission antenna 62. In the modulation device 100, the references 10, 11a and 12 denote the crystal oscillator, the additional capacitor and the switching relay for the latter. The circuit 1, of the Colpitts oscillator type, is designed to produce a signal of frequency 13.5225 MHz, and this frequency is slightly reduced when the capacitor 11a is connected by the relay 12. The amplifier 61 is of the rectifier type, such that the frequency of the RF transmission signal is multiplied by 2 at the antenna 62. The RF transmission channel used in this example is then 2×13.5225=27.045 MHz.
However, such a frequency-shift modulation device has the following drawbacks. First of all, the crystal oscillator 10 of the oscillating circuit 1 must have a frequency which is precisely defined with respect to the RF channel used. It is then necessary to change the model of the crystal oscillator 10 in order to vary the RF channel. For this reason, a transmission device with a variable RF channel must incorporate several oscillating circuit crystals which are alternately connected depending on the channel selected. Such a variable-channel device is particularly complex and costly. Furthermore, the crystal oscillators required are not models that are widely commercially available, so that they are each costly in themselves. Lastly, the characteristics of the crystal oscillators used and those of the additional capacitor are likely to vary within the same production runs of these components. This results in deviations of the frequency difference which is used for the coding of the transmitted RF signal. The compatibility of the device with a given RF receiver may then be affected.
Other frequency-shift modulation devices incorporate fractional phase-locked loop (PLL) circuits, or alternatively high-frequency digital-analog converters (DAC), in order to use oscillating circuit crystals that have no specific frequency imposed by the RF channel used, and that are widely available on the market. However, such fractional PLL circuits and such DACs are themselves costly, notably because of the numerous analogue components that they incorporate. In addition, they have high electrical power consumptions.
There is a need in the art to provide a frequency-shift modulation device that does not have the aforementioned drawbacks.